This invention relates to an optical element, such as a membrane or pellicle, and more particularly to a plural-layer, composite, high-transmissivity element with a two-layer, organic, anti-reflective coating.
In recent years, various optical elements, such as pellicles, have played an important role in the making of semiconductor wafers that are used in various integrated semiconductor circuits. As is well understood by those skilled in the art, pellicles protect (against contamination) photo masks which are used in the various photolithography steps required in wafer preparation.
The typical pellicle takes the form of an extremely thin optical film which is supported on a ring-like frame, the entirety which is then placed over a wafer during photolithography. In order to be most effective, a pellicle, in addition to preventing contamination of a photo mask, should also exhibit a very high degree of optical transmissivity at the wavelength of light which is used during photolithography.
While single-layer pellicles, typically made of nitrocellulose, have been widely and successfully used in the past, there are many instances in which the user desires an even higher degree of transmissivity than is obtainable with such a pellicle. To this end, the preparation of plural-layer pellicles where so-called anti-reflective coatings are added have been proposed. Such coatings, in the past, in the field of pellicles, have taken the form of multiple layers of inorganic material suitably deposited, one after another, on one face, or on both faces, of a base nitrocellulose layer. Such coatings, however, have presented several problems.
To begin with, materials chosen in the past to prepare anti-reflective coatings have required vacuum deposition at extremely high temperatures, such as above 1,000-degrees Centigrade, and thus have dictated a relatively slow and expensive batch-processing manufacturing approach. Another important concern is that a significant thermal mismatch exists between the usual base nitrocellulose layer and such anti-reflective coating layers which can result in cracking and/or subsequent peeling of the anti-reflective coating.
A general object of the present invention, therefore, is to provide a unique optical element construction including an anti-reflective coating which avoids the difficulties just mentioned.
According to a preferred embodiment of the invention, the proposed element (usable as a pellicle) includes a base layer, formed typically of nitrocellulose or cellulose acetate, an intermediate layer formed of an organic material which is joined to and distributed over a face in the base layer, and another layer formed of another organic material joined to and distributed over the exposed face in the intermediate layer. The intermediate layer proposed in the construction of the invention takes the form of a aromatic polymer compound, or a vinyl-group-containing compound. Well suited for this layer are polyvinylnaphthalene, polymethylstyrene, and polystyrene, prepared for distribution over the nitrocellulose or cellulose acetate layer in a solvent of toluene or xylene. The third-mentioned layer takes the form of a fluorocarbon compound, the composition of which, in a preferred form, prepared in a suitable solvent, will be described below.
The various objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the single drawing figure.